1. Field of the Invention
The present invention pertains to a method and apparatus for determining the cardiac output of a patient, and, more particularly, to a method of determining cardiac output by analyzing the effect that an induced change in the patient's arterial oxygen concentration has on their oxygen uptake and fractional arterial oxygen concentration, and to an apparatus for use in implementing such a method.
2. Description of the Related Art
There are several generally accepted techniques for measuring cardiac output (CO), which is the total volumetric flow of blood through the heart, and, thus, through the body at any given time. These techniques include: thermodilution, dye dilution, the direct Fick method, and partial CO2 rebreathing. Thermodilution involves injecting cold saline directly into the right atrium of the heart and measuring the temperature change downstream in the pulmonary artery using a temperature sensor placed in this artery. Cardiac output is determined based on this temperature change versus time. Dye dilution is similar to thermodilution except that a dye, rather than cold saline, is injected into the pulmonary artery. The amount of dye collected downstream is measured to determine the patient's cardiac output.
According to the direct Fick method, either the content of oxygen (O2) or the content of carbon dioxide (CO2) in both the arterial blood and mixed venous blood are measured. The Fick equation, written for oxygen, is: CO=O2 uptake/(the content of O2 in arterial blood−the content of O2 in mixed venous blood). The Fick equation, written for carbon dioxide, is: CO=CO2 excreted/(the content of CO2 in mixed venous blood−the content of CO2 in arterial blood). As noted above, the direct Fick method requires obtaining a mixed venous blood sample, which is only available in the pulmonary artery. See FIG. 1.
It can thus be appreciated that thermodilution, dye dilution, and the direct Fick method for determining cardiac output all require insertion of a catheter into the patient at, near, or through the heart. More specifically, in implementing these cardiac output measurements, a catheter is usually floated through the chambers of the heart in order to insert the saline or dye or to obtain the necessary blood sample at the correct location. For this reason, either of the above cardiac output measurement techniques is very invasive. Indeed, it is known that an arrhythmia may result from the placement of the catheter in or through the heart. Therefore, these cardiac output measurement techniques are typically only performed in the most critical of situations, where the need to know the patient's cardiac output outweighs the risk to the patient in taking this measurement.
The partial CO2 rebreathing technique for measuring cardiac output, on the other hand, is a noninvasive approach believed to have been developed by Novametrix Medical Systems, Inc. of Wallingford, Conn. (Novametrix). This method is implemented using a device referred to as a NICO™ sensor, which is distributed by Novametrix. The NICO sensor measures the flow of gas to and from the patient and the CO2 content in the patient's exhaled gas.
The partial CO2 rebreathing cardiac output measurement technique is based on the CO2 Fick equation in conjunction with what is called partial CO2 rebreathing. According to this partial CO2 rebreathing technique, cardiac output is measured by comparing the patient's CO2 excretion to the arterial CO2 content during normal breathing and during rebreathing, in which the patient rebreathes expired gases for a period of time. Cardiac output is determined as: CO=the change in CO2 excretion/the change in the arterial CO2 content.
Arterial CO2 is typically determined from a sample of arterial blood. However, in order to eliminate the need for a blood sample to measure the arterial CO2 content, the partial CO2 rebreathing technique substitutes end tidal CO2 (ETCO2) for the required arterial CO2 measurement. Therefore, the cardiac output equation becomes: CO=the change in CO2 excretion/the change in the ETCO2.
This partial CO2 rebreathing technique, however, has several disadvantages. Namely, the patient is preferably intubated or breathing through a trachea tube when taking the flow and CO2 measurements to capture the total volume of CO2. In addition, the patient must be heavily sedated or unconscious so that he or she is not breathing spontaneously. If the patient is breathing spontaneously, the increased CO2 level in the blood during the rebreathing phase would automatically trigger the patient's respiratory system to speed up or deepen the breaths to remove the excess CO2. It is well known that for most patient's the level of CO2 is more important than the level of O2 as a mechanism for triggering ventilation. Such a change in rate or depth of breathing prevents an accurate determination of cardiac output under this technique. It should also be noted that the use of end tidal CO2, as opposed to the arterial CO2 content, may introduce errors in determining cardiac output, because there are situations where the end tidal CO2 may not correlate with the arterial CO2 content. The partial CO2 rebreathing cardiac output measurement technique is also disadvantageous because it does not adequately account for shunt blood flow, which is blood that is not oxygenated during the respiratory cycle. This flow cannot be directly measured, but must be estimated when using this conventional cardiac output measurement technique.